Use of dilute steam ambient for improvement of flash devices

ABSTRACT

The present invention provides a flash memory integrated circuit and a method for fabricating the same. The method includes etching a gate stack that includes an initial oxide layer directly in contact with a silicon layer, defining an oxide-silicon interface therebetween. By exposing the etched gate stack to elevated temperatures and a dilute steam ambient, additional oxide material is formed substantially uniformly along the oxide-silicon interface. Polysilicon grain boundaries at the interface are thereby passivated after etching. In the preferred embodiment, the interface is formed between a tunnel oxide and a floating gate, and passivating the grain boundaries reduces erase variability due to enhanced charge transfer along grain boundaries. At the same time, oxide in an upper storage dielectric layer (oxide-nitride-oxide or ONO) is enhanced in the dilute steam oxidation. Thermal budget can be radically conserved by growing thin oxide layers on either side of a nitride layer prior to etching, and enhancing the oxide layers by dilute steam oxidation through the exposed sidewall after etching. The thin oxide layers, like the initial tunnel oxide, serve as diffusion paths to enhance uniform distribution of OH species across the buried interfaces being oxidized.

FIELD OF THE INVENTION

The present invention generally relates to transistor gate dielectricsand methods of fabricating the same. More particularly, the inventionrelates to processes and structures for improving tunnel oxide qualityin erasable programmable read-only memories (EEPROMs).

BACKGROUND OF THE INVENTION

Memory devices such as erasable programmable read-only memories(EPROMs), electrically erasable programmable read-only memories(EEPROMs), or flash erasable programmable read-only memories (FEPROMs)are erasable and reusable memory cells which are often used in digitalcellular phones, digital cameras, LAN switches, cards for notebookcomputers, etc. A memory cell operates by storing electric charge(representing an “on” state) on an electrically isolated floating gate,which is incorporated into a transistor. This stored charge affects thebehavior of the transistor, thereby providing a way to read the memoryelement. It is therefore crucial that the memory cell be able tomaintain the stored charge over time, so that charge leakage does notcause data errors by converting “on” states to “off.”

A flash memory cell typically consists of a transistor, a floating gate,and a control gate above the floating gate in a stacked gate structure.The floating gate, typically composed of polycrystalline silicon (i.e.,“polysilicon”), is electrically isolated from the underlyingsemiconductor substrate by a thin dielectric layer, which is typicallyformed of an insulating oxide, and more particularly, silicon oxide.Because charge is transferred across the dielectric layer byquantum-mechanical tunneling, this dielectric layer is often referred toas a “tunnel oxide” layer. Such tunnel oxide layers are typicallyapproximately 100 Å thick. Properties of the tunnel oxide must bestrictly controlled to ensure the ability to read and write bytunneling, while avoiding data loss through charge trapping or leakage.The control gate is positioned above the floating gate, and iselectrically isolated from the floating gate by a storage dielectriclayer, such as oxide-nitride-oxide (ONO). Electrical access to thefloating gate is therefore only through capacitors.

Storing charge on the floating gate programs a memory cell. This isachieved via hot-electron injection by applying a high positive voltage(approximately 12 V) to the control gate, and a high drain-to-sourcebias voltage (approximately 6 V). An inversion region is created betweenthe source and drain by the control gate voltage, and electrons areaccelerated from the source to the drain by the drain bias voltage. Somefraction of these electrons will have sufficient energy to surmount thetunnel oxide barrier height and reach the floating gate. The floatinggate is therefore programmed by collecting and storing these electronsto represent an “on” state.

An EPROM device can be erased (i.e., returned to an “off” state) byexposing the floating gate to ultraviolet light, which excites thestored electrons out of the floating gate. The erasure of an EEPROM orFEPROM cell is accomplished via Fowler-Nordheim tunneling, in which anelectric field is applied which is sufficient for the stored electronsto traverse the tunnel oxide and enter the substrate, thereby reducingthe stored charge in the floating gate. Under this mechanism fordischarging the floating gate, a large negative voltage (e.g., −10 V) isapplied to the control gate, and a positive voltage (e.g., 5-6 V) isapplied to the source while the drain is left floating. Electrons thentunnel from the floating gate through the tunnel oxide, and areaccelerated into the source. Because both the programming and erasing ofa memory element takes place via charge transfer processes across thetunnel oxide layer, it is important to minimize the density of interfacestates and other defects in the form of charge traps in this regionwhich would otherwise create a mechanism for charge trapping or leakagethrough the tunnel oxide.

Once the stacked gate structure has been fabricated and etched to theappropriate dimensions, the stacked gate structure is encapsulated in aliner layer, followed by the formation of an insulating layer, typicallycomposed of thick, planarized borophosphosilicate glass (BPSG). Theliner layer between the source/drain regions and the BPSG, oftencomposed of a low pressure chemical vapor deposition (LPCVD) oxide,serves to minimize out-diffusion of contaminants and dopants from theBPSG. Such out-diffusion might otherwise affect the performance ofunderlying devices.

While processes have been developed to improve gate dielectric quality,as measured by improved data retention in flash memory devices, forexample, a finite soft error rate remains. Accordingly a need exists forfurther improvements in the fabrication of transistor gate dielectrics.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, a method isprovided for fabricating an integrated circuit. The method includesproviding a silicon semiconductor substrate and forming a stacked gatestructure comprising an oxide-silicon interface defined by an initialoxide layer directly in contact with a silicon layer. Upon patterningthe stacked gate structure to define a gate stack that includes theoxide-silicon interface, a final oxide layer is formed with additionalsilicon oxide bonds in the region of the oxide-silicon interface. Thisresult is achieved by exposing the patterned gate stack to elevatedtemperatures and a dilute steam ambient.

In accordance with another aspect of the invention, a method is providedfor fabricating an integrated circuit. The method includes providing asemiconductor substrate and forming a transistor gate stack thatincludes an oxide-silicon interface between an initial oxide layer andan overlying polycrystalline silicon layer. The transistor gate stack isetched to define a gate electrode that includes the oxide-siliconinterface. Polycrystalline silicon grain boundaries are then passivatedacross the oxide-silicon interface by exposing the grain boundaries toOH species.

In accordance with yet another aspect of the invention, a method isprovided for fabricating an integrated circuit. The method includesforming a plurality of layers over a semiconductor substrate, includinga buried oxide layer having an interface with a silicon source layer.The plurality of layers is etched to expose a surface of the buriedoxide layer. Then, OH species diffuse through the surface across theinterface with the silicon source layer to grow additional oxide.

In accordance with yet another aspect of the invention, an integratedcircuit includes a transistor with a polysilicon layer directly incontact with an oxide layer, forming an oxide-polysilicon interfacebetween these layers. Along the interface, polysilicon grain boundariesinclude oxide bonds.

In the illustrated embodiments, a stacked gate structure is formed onthe surface of a silicon semiconductor substrate, wherein the stackedgate structure comprises an initial tunnel dielectric layer, one or morepolysilicon layers, and an initial storage dielectric layer comprisingone or more initial oxide layers. After patterning, the stacked gatestructure is exposed to elevated temperatures and a dilute steam ambientcomprising steam and hydrogen. This dilute steam ambient process yieldsOH radicals which readily diffuse through the initial oxide and formadditional oxide material substantially uniformly along theoxide-silicon interfaces of the stacked gate structure. The oxidationrate is controlled by diluting the steam with carrier and preferablyhydrogen-bearing gas, and by keeping the oxidation temperature moderate.As a result, oxidation is slower relative to the lateral diffusion of OHspecies from the gate sidewalls across the entire interface. The finaltunnel dielectric layer thus has a substantially uniform thickness. Mostpreferably, the oxide thickness of a storage dielectric layer issimultaneously increased.

Advantageously, the exposure of the stacked gate structure to the dilutesteam ambient improves the performance of the resulting devicestructures. First, the number of defects, such as oxygen vacancies anddangling bonds, in the region of the final tunnel dielectric layer isreduced by the additional oxide material. Second, consumption of thermalbudget can be dramatically reduced due to the relatively lowtemperatures and time of the dilute steam ambient oxidation in forming asubstantial fraction of the final storage dielectric layer. And third,the grain boundaries of polysilicon layers within the stacked gatestructure are passivated during the dilute steam oxidation, therebyreducing the effect of the grain boundaries on the erase characteristicsof the resulting devices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates an interim stage of the fabrication ofthe stacked gate structure with a nitride layer on top of a lower oxidelayer, in accordance with prior art methods.

FIGS. 2A and 2B schematically illustrate the stacked gate structure ofFIG. 1 after formation of an upper oxide layer on top of the nitridelayer.

FIG. 3 schematically illustrates the stacked gate structure of FIG. 2Aafter formation of a control gate and a cap insulator layer, inaccordance with the prior art.

FIG. 4 schematically illustrates the stacked gate structure of FIG. 3after patterning and etching to define.

FIGS. 5A and 5B schematically illustrate the fabrication of the stackedgate structure of FIG. 4 after a prior art source/drain reoxidation.

FIG. 6 is a flow chart, generally illustrating a process flow inaccordance with a preferred embodiment of the present invention.

FIGS. 7A and 7B schematically illustrate a partially fabricated stackedgate structure, constructed in accordance with a preferred embodiment ofthe present invention, including an initial gate dielectric and aninitial storage dielectric.

FIG. 8 schematically illustrates the stacked gate structure of FIG. 7Aafter formation of a control gate and a cap insulator layer.

FIG. 9 schematically illustrates the stacked gate structure of FIG. 8after patterning and etching, but before exposure to a dilute steamambient.

FIGS. 10A to 10C schematically illustrate the patterned gate of FIG. 9after the exposure to a dilute steam ambient.

FIG. 11 schematically illustrates the polysilicon grain boundaries atthe gate dielectric-polysilicon interface after exposure to a dilutesteam ambient.

FIG. 12 schematically illustrates the patterned gate of FIG. 10A afterformation of spacers and further insulating layers over the gateelectrode.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

While illustrated in the context of an electrically erasableprogrammable read only memory (EEPROM) device for flash memory circuits,persons skilled in the art will readily find application for the presentinvention to fabrication of other semiconductor integrated circuitdevices. In particular, methods disclosed herein are applicable toimproving dielectric-conductor interfaces in a wide variety of devicedesigns with a wide variety of process flows. The methods describedherein, however, have particular utility for improving the performanceof dielectric layers in flash memory gate stacks.

FIG. 1 illustrates an interim structure during the fabrication of anEEPROM transistor 10 in a flash memory cell constructed using prior artmethods. The EEPROM transistor 10 includes a stacked gate structure 26fabricated over a semiconductor substrate 20, which in the illustratedembodiments (and the prior art figures) is formed from the upper portionof a single-crystal silicon wafer. The interim stage of the stacked gatestructure 26 illustrated in FIG. 1 includes a tunnel dielectric layer30, a floating gate 50, and a partially formed storage dielectric layer60 that includes a lower oxide layer 62 and a nitride layer 64.

The fabrication of the interim stacked gate structure 26 begins with theformation of a tunnel dielectric layer 30 by blanket formation acrossthe whole exposed surface of the substrate 20. The tunnel dielectriclayer 30 typically comprises an oxide, and more particularly silicondioxide formed by thermal oxidation of the substrate surface. The tunneldielectric layer 30 is formed to a thickness of approximately 100 Å.

The layers of the stacked gate structure 26 can be completed in anysuitable fashion, typically entailing numerous processing steps. Theformation of the floating gate 50 on the tunnel dielectric layer 30 isachieved by depositing a conductive layer (typically doped polysilicon)onto the tunnel dielectric layer 30. Doping of the polysilicon of thefloating gate 50 can be either in situ (i.e., while the floating gate 50is being formed) or it can be a separate step after the polysilicondeposition. Persons skilled in the art are able to select appropriatematerials and methods for creating the floating gate 50 with aparticular set of characteristics.

The formation of the stacked gate structure 26 continues by theformation of a storage dielectric layer 60 on the floating gate 50. Anexemplary storage dielectric layer 60 is composed of oxide-nitride-oxide(ONO). Alternatively, high dielectric materials may be employed in thestorage dielectric layer 60 to improve the capacitance of the EEPROMdevice. Persons skilled in the art can readily select appropriatematerials for the storage dielectric layer 60 for particular circuitdesigns.

The formation of the ONO storage dielectric layer 60 as illustrated inFIGS. 1, 2A and 2B comprises formation of a lower oxide layer 62, anitride layer 64 on top of the lower oxide layer 62, and an upper oxidelayer 66 on top of the nitride layer 64. The lower oxide layer 62 istypically formed by thermal oxidation at the upper surface of thefloating gate 50 until this sub-layer reaches its desired finalthickness (e.g., 40 Å to 50 Å thick).

The formation of the nitride layer 64 is typically accomplished by lowpressure chemical vapor deposition (LPCVD), but other fabricationprocesses to form the nitride layer 64 may be used. Typically, thethickness of the formed nitride layer 64 is approximately 70 Å, asillustrated in FIG. 1.

After formation of the nitride layer 64, the upper oxide layer 66 isformed on top of the nitride layer 64, as illustrated in FIGS. 2A and2B. Growth of the upper oxide layer 66 is typically achieved by thermaloxidation at the top of the nitride layer 64, which requires relativelyhigh temperatures and long processing times. The upper oxide layer 66 isformed at the expense of a portion of the nitride layer 64, which ispartially consumed during oxidation. For example, formation of an upperoxide layer 66 that is 40 Å thick will consume approximately 40 Å of thenitride layer 64. Therefore, an initial nitride layer thickness of 70 Åis required to form an upper oxide layer 66 that is 40 Å thick and aresultant nitride layer thickness of 30 Å, as illustrated in FIGS. 2Aand 2B. Typically, this process requires heating the substrate 20 toapproximately 1,000° C. in an ambient of H₂O and O₂ and at a pressure ofapproximately 760 Torr for a period of approximately 4 hours to 6 hoursin a typical furnace oxidation.

Referring to FIG. 3, the formation of the gate stack then continues withthe formation of a control gate 70 over the storage dielectric layer 60and a cap insulator layer 80 on top of the control gate 70 to completethe gate stack. The control gate 70 is composed of polysilicon, however,various other conductive materials may be used, including but notlimited to, metals (e.g., tungsten) and metal silicides. The capinsulator layer 80 typically comprises an insulator such as siliconnitride or silicon oxide.

As illustrated in FIG. 4, the stacked gate structure is then patterned,typically by conventional photolithography and etch processes, to definea patterned gate electrode 85. The etching which forms the stacked gatestructure 26 stops approximately at the tunnel dielectric layer 30 oversource and drain regions on either side of the gate 85. Damage from theetch is thereafter repaired in a source/drain reoxidation, typically byexposure to dry oxygen at high temperatures. The resulting structure,illustrated in FIGS. 5A and 5B, exhibits some “smile” at theoxide-silicon interface, or gate corner rounding. “Smile” is the termfor structures in which the edges of an oxide layer are thicker than thecenter of the oxide layer. The thicker oxide at the edges of the tunneldielectric layer 30 and the rounding of the bottom corners of thefloating gate 50 result from the low diffusion rate and high reactionrate of O₂. Both the low diffusion rate and the high reaction rateincrease the probability that an O₂ molecule diffusing along theoxide-silicon interface will form an oxide species before it reaches thecenter of the interface, such that the edges of the oxide layer becomethicker than the center.

The elevated temperatures and long processing times associated with theformation of the gate stack, particularly during the formation of theupper oxide layer 66, have a deleterious effect on previously fabricatedcomponents of the EEPROM transistor 10. For example, it is important toavoid reduced charge mobilities in the region of the tunnel dielectriclayer 30, particularly in flash memory devices which utilize a completechannel erase. Exposure to elevated temperatures and long processingtimes create various forms of charge traps (e.g., vacancies and danglingbonds) in the region of the tunnel dielectric layer 30. In order tominimize the effects of such charge traps in the performance of thefabricated devices, it is desirable to be able to repair or passivatethe charge traps created in the region of the tunnel dielectric layer 30during the fabrication process. Additionally, it is generally desirableto minimize the time the device is exposed to elevated temperaturesduring its fabrication (i.e., to conserve the “thermal budget”) tothereby reduce the number of charge traps created.

FIG. 6 is a flow chart which generally illustrates a process flow inaccordance with one preferred embodiment of the present invention, andFIGS. 7A to 12 illustrate various stages of the fabrication of an EEPROMtransistor in accordance with this preferred embodiment. In thefollowing description of the preferred embodiment, the named processflow steps are found in FIG. 6 and the numbered structural elementsrefer to FIGS. 7A-12. It will be understood, however, that elements maydiffer in appearance during fabrication as compared to the illustratedfinal structure.

As in the prior art methods, the EEPROM transistor 10 produced by thepreferred embodiment of the present invention is fabricated over asemiconductor substrate 220. FIG. 6 includes providing 100 such asemiconductor substrate 220. In the illustrated embodiment, thesubstrate 220 comprises the upper portion of a single-crystal siliconwafer. In general, however, the substrate can comprises anysemiconductor structure or layer in which the lowest level of integratedelectrical devices are formed.

The fabrication of the gate stack begins with the formation 110 of aninitial tunnel dielectric layer 230 across the whole exposed surface ofthe substrate 220. The thickness of this initial tunnel dielectric layer230 is less than the desired final thickness. The initial tunneldielectric layer 230 preferably comprises an oxide, and moreparticularly silicon oxide, though the skilled artisan will appreciatethat the present invention will have utility in conjunction with othertypes of oxide. An exemplary alternative oxide comprises tantalumpentoxide (Ta₂O₅). In the illustrated embodiment, formation 110 of theinitial tunnel dielectric layer 230 comprises thermal oxidation of thesubstrate surface, but persons skilled in the art are able to select anappropriate method of forming the initial tunnel dielectric layer 230from the various possible methods. The thickness of the illustratedinitial tunnel dielectric layer 230 after this step in the fabricationof the stacked gate structure 26 is preferably between about 6 Å and 94Å, (e.g., for a flash memory device with a final tunnel dielectricthickness of 100 Å). More generally, the initial tunnel dielectric layer230 is grown to a thickness sufficient to provide a diffusion path forOH species. Preferably it has a thickness between about 5% and 95% ofthe thickness of the desired final thickness for this layer, morepreferably between about 40% and 90% of the thickness of the desiredfinal thickness, and most preferably between about 70% and 90% of thedesired final thickness.

The remaining layers of the gate stack can be completed in any suitablefashion, typically entailing numerous processing steps. In theillustrated embodiment, formation 120 of a floating gate 250 is achievedby depositing a conductive layer (preferably doped polysilicon) onto theinitial tunnel dielectric layer 230. As with the prior art method, thedoping of the polysilicon of the floating gate 250 can be either in situ(i.e., while the floating gate 250 is being formed) or it can be aseparate step after the polysilicon deposition. Persons skilled in theart are able to select appropriate materials and methods compatible withthe present invention for creating the floating gate 250 with aparticular set of characteristics.

The formation of the gate stack structure 26 of the preferred embodimentcontinues with at least partial formation 130 of a storage dielectriclayer 260 on the floating gate 250. In the illustrated embodiment, aninitial storage dielectric layer 260 includes an oxide and isparticularly composed of oxide-nitride-oxide (ONO). Significantly, thisinitial layer 260 is originally formed to have a thickness which is lessthan the thickness of the desired final thickness. Alternatively, highdielectric materials may be employed in the storage dielectric toimprove the capacitance of the EEPROM device. Persons skilled in the artcan readily select appropriate materials for the storage dielectriclayer for particular circuit designs. In other embodiments of thepresent invention in which the storage dielectric contains no oxidelayers, the storage dielectric layer can be completely formed with itsdesired final thickness.

As best seen from FIG. 7B, the formation 130 of the initial storagedielectric layer 260 in the illustrated embodiment comprises formationof an initial lower oxide layer 262, formation of a nitride layer 264 ontop of the initial lower oxide layer 262, and formation of an initialupper oxide layer 266 on top of the nitride layer 264. The thickness ofthe initial lower oxide layer 262, is less than the desired finalthickness. Similarly, the thickness of the initial upper oxide layer 266is less than the desired final thickness.

In the illustrated embodiment, the formation of the initial lower oxidelayer 262 comprises thermal oxidation at the upper surface of thefloating gate 250. Alternatively, the initial lower oxide layer 262 canbe deposited onto the floating gate 250 by CVD, for example. Thermaloxidation growth of an initial lower oxide layer 262 of 20 Å comprisesheating the workpiece to approximately 650° C. in an ambient of O₂ at apressure of approximately 760 Torr for a period of between about 2minutes and 5 minutes. However, persons skilled in the art are able toselect alternative values of these parameters or alternative methods offorming the initial lower oxide layer 262, depending upon the desiredinitial thickness.

The thickness of the initial lower oxide layer 262 is chosen based uponthe desired final thickness, but it is preferably between approximately15 Å and 25 Å for a desired final lower oxide thickness of 40 Å. Moregenerally, the thickness of the initial lower oxide layer 262 issufficient to serve as a diffusion path for OH species during a laterreoxidation, which requires only about 1 or 2 monolayers. Preferably,the thickness of the initial lower oxide layer 262 is betweenapproximately 5% and 95% of the desired final thickness, more preferablybetween approximately 40% and 60% of the desired final thickness forthis layer.

The formation of the silicon nitride layer 264 is accomplished by lowpressure chemical deposition vapor (LPCVD), but other embodiments whichuse other fabrication processes to form the nitride layer 264 are alsocompatible with the present invention. In the illustrated embodiment,the thickness of the nitride layer at the stage illustrated in FIG. 1 ispreferably between approximately 40 Å and 100 Å, and more preferablybetween approximately 65 Å and 75 Å. More generally, the thickness ofthe nitride layer 264 at this stage in the fabrication is preferablygreater than approximately 150% of the thickness of the nitride layer 64after the formation of the initial upper oxide layer 266, and morepreferably greater than approximately 170% of the thickness of thenitride layer 264 after the formation of the initial upper oxide layer266.

After formation of the nitride layer 264, the illustrated embodimentincludes formation of the initial upper oxide layer 266. In theillustrated embodiment, growth of the initial upper oxide layer 266 isachieved by thermal oxidation at the top of the nitride layer 264, whichrequires relatively high temperatures and long processing times. Theinitial upper oxide layer 266 is formed at the expense of the nitridelayer 264, which is paritally consumed during oxidation. For example,formation of an initial upper oxide layer 266 that is 20 Å thick willconsume approximately 10 Å of the nitride layer 264. Therefore, aninitial nitride layer thickness of 40 Å is required to form an initialupper oxide layer 266 that is 20 Å thick and a resultant nitride layerthickness of about 30 Å. Thermal oxidation growth of an initial upperoxide layer 266 of approximately 20 Å thickness can be readilydetermined by the skilled artisan. Preferably, the prior art processusing H₂O and O₂ can be adjusted by lowering the temperature and/orduration of the oxidation. Alternatively, other values of theseparameters, or other methods of fabrication of the initial upper oxidelayer 266 are compatible with the present invention. The thickness ofthe initial upper oxide layer 266 is chosen based upon the desired finalthickness of this layer, but it is preferably between approximately 10 Åand 40 Å, and more preferably between approximately 15 Å and 25 Å. Moregenerally, the initial upper oxide layer 266 is provided in a thicknesssufficient to serve as a diffusion path for OH species. Preferably, ithas a thickness between approximately 5% and 95% of the desired finalthickness, and more preferably between approximately 40% and 60% of thedesired final thickness for this layer.

The formation of the gate stack then continues by the formation 140 of acontrol gate 270 over the initial storage dielectric layer 260. In theillustrated embodiment, the control gate 270 is composed of polysilicon,however, in other embodiments the control gate 270 can be composed ofvarious other conductive materials, including, but not limited to, metal(e.g., tungsten) and/or metal silicide. Upon formation 140 of thecontrol gate 270, a cap insulator layer 280 is preferably formed 150over the control gate 270, as illustrated in FIG. 8. The cap insulatorlayer 280 comprising an insulator such as silicon nitride or siliconoxide.

Referring to FIG. 9, the gate stack is then patterned 160, such as byconventional photolithography and etch processes, to define a gateelectrode 285. As noted above, vertical etching typically causes somedamage to the source/drain regions and the tunnel oxide at the corner ofthe gate 285.

In the preferred embodiment of the present invention, the patterning ofthe gate stack is followed by a dilute steam oxidation 170 performedunder conditions which form oxide layers at existing oxide-siliconinterfaces within the patterned gate electrode 285, with each resultingoxide layer having a substantially uniform thickness across the entireinterface. This is accomplished by exposing the patterned gate electrode285 to elevated temperatures in an ambient which yields OH radicals. Inthe preferred embodiment of the present invention, this ambientcomprises dilute steam (H₂O) in a hydrogen flow. However, personsskilled in the art are able to select other chemical constituents whichyield OH radicals and are compatible with the present invention. Forexample, H₂O₂ may be used in place of steam, and numeroushydrogen-containing compounds may be used in place of hydrogen (e.g.,NH₃, N₂H₄, or hydrazine). In some arrangements, remote or in situ plasmacan also generate OH radicals. The OH radicals react with the siliconatoms at the oxide-silicon interface, converting the silicon atoms intoadditional oxide species.

Use of dilute steam ambients at relatively low temperatures can resultin oxide growth conditions at oxide-conductor interfaces that have areaction rate limited growth region even for very long diffusionlengths. In particular, OH radicals can quickly diffuse through existingoxide layers, producing a uniform oxidant supply across theoxide-conductor interface. For example, on 0.18 μm wordline flash memorystructures, the preferred embodiment of the present invention can grow asubstantially uniform oxide underneath the floating gate 250 withsubstantially no “smile.”

As compared to a standard reoxidation step, in which the substrate 220is exposed to dry oxygen resulting in a “smile” structure, the use ofdilute steam ambients to form additional oxide material at anoxide-conductor interface avoids substantial smile due in part to thehigher diffusion rate of the OH species from the steam ambient, relativeto O₂, and in part to the high dilution which lowers the growth rate. Inthis way, OH species can diffuse to the center of the oxide-siliconinterface to grow additional oxide there. This is in contrast to thestandard source/drain reoxidation process which utilizes free oxygen andwhich is unable to form oxide material toward the center of theoxide-silicon interface, and which is deliberately arranged fordisproportionate growth. The addition of stable hydrogen compounds tothe ambient substantially avoids the creation of other oxygen species(e.g., O or O₂) which would otherwise contribute to disproportionateoxide growth at the corners (“smile”). Hydrogen compounds reduce thecreation of free oxygen species by providing hydrogen atoms, whichmaintain the OH species in the dilute steam ambient.

During the dilute steam ambient oxidation 170 of the preferredembodiment of the present invention, the patterned gate electrode 285 isexposed to an ambient comprising steam, hydrogen, and an inert carriergas such as Ar or N₂. The inert carrier serves to dilute H₂O in order tobalance the rate of OH diffusion and OH reaction rate to achievesubstantially uniform regrowth. In one embodiment of the presentinvention, the steam is generated by a catalytic process in which O₂molecules impinge upon hydrogen atoms which are bonded to the surface ofa catalytic metal surface at an elevated temperature (e.g.,approximately 400° C.). Alternatively, the steam may be generated by apyrogenic process, or by a bubbler. Persons skilled in the art are ableto select an appropriate source of steam compatible with the presentinvention.

Conditions are arranged to moderate the rate of oxidation, relative tothe rate of OH diffusion from the gate sidewalls across thedielectric-conductor interface. The pressure of the chamber during thedilute steam ambient oxidation 170 is relatively inconsequential, but ispreferably higher than 35 mTorr, and more preferably betweenapproximately 100 Torr and 800 Torr. The partial pressure of steam inthis ambient is preferably between approximately 8 Torr and 680 Torr,more preferably between approximately 40 Torr and 160 Torr. Moregenerally, the percentage of the ambient that is H₂O is preferablybetween approximately 0.1% and 99%, more preferably betweenapproximately 1% and 50%, and most preferably less than about 10%. Thetemperature of the workpiece during the dilute steam ambient oxidation170 is preferably between approximately 500° C. and 1,000° C., and morepreferably between approximately 600° C. and 800° C., and mostpreferably between approximately 630° C. and 670° C. As will be readilyappreciated by the skilled artisan, in view of the present disclosure,higher temperatures can be compensated by lower steam partial pressures,and vice versa, in order to ensure moderate oxidation rates.

The skilled artisan can readily determine the length of steam exposurerequired for the desired additional oxide thickness.

FIGS. 10A to 10C schematically illustrate the resulting structure afterthe dilute steam ambient oxidation 170. Under the preferred conditions,additional oxide is formed at each oxide-silicon interface with asubstantially uniform thickness across the entire interface (i.e., nosmile), producing a final gate oxide 230′ with an enhanced thickness andpassivated grain boundaries. The amount of additional oxide formation isdependent on the exposure time of the dilute steam ambient oxidation 170and on the other process parameters such as pressure and temperature.The difference between the maximum and minimum thickness of the finaltunnel dielectric layer 230′ is preferably no more than about 8 Å, morepreferably no more than about 4 Å.

In an exemplary dilute steam oxidation, an initial tunnel dielectriclayer 230 of approximately 90 Å is formed using one of the standardtechniques known to persons skilled in the art. After formation andpatterning of the rest of the gate stack, the device is held at anelevated temperature of about 650° C. while being exposed to an ambientof approximately 760 Torr with 10% steam and 90% hydrogen. Afterapproximately 30 minutes of this dilute steam ambient oxidation 170,approximately 10 Å to 20 Å of additional oxide is formed resulting inthe final tunnel dielectric layer 230′ of 100 Å to 110 Å withsubstantially no smile.

Concurrently with the growth of additional oxide at the initial tunneldielectric layer 230, in the preferred embodiment of the presentinvention, the dilute steam ambient oxidation 170 yields additionaloxide at other oxide-silicon interfaces of the gate electrode 285 (e.g.,the oxide layers of the ONO initial storage dielectric layer 260). Forexample, after forming an initial storage dielectric layer 260comprising an initial lower oxide layer 262 of approximately 20 Å, anitride layer 264 of approximately 30 Å, and an initial upper oxidelayer 266 of approximately 20 Å, the dilute steam ambient oxidation 170using the above-described set of parameters adds approximately 20 Å toeach oxide-silicon interface during the process. The resulting finalstorage dielectric layer 260′ then has a lower oxide layer 262′ of 40 Å,a final nitride layer 264′ of 30 Å, and an upper oxide layer 266′ of 40Å.

In other embodiments of the present invention, larger fractions of thefinal tunnel dielectric layer 230′ or the final storage dielectric layer260′ may be formed using the dilute steam ambient oxidation 170. Asnoted, the initial oxide layers are preferably between about 5% and 95%of the respective final desired thicknesses. To form additional oxideduring the dilute steam ambient oxidation 170, the previously fabricatedinitial oxide layer serves as the diffusion path for the OH species. OHradicals do not diffuse through silicon sufficiently fast to produceoxide layers with substantially uniform thicknesses.

In certain embodiments of the present invention, the use of the dilutesteam ambient oxidation 170 to form a fraction of the final storagedielectric layer 260′ represents a substantial savings of the thermalbudget associated with the formation of the gate electrode 285.Conventional formation of a 40 Å thick upper oxide layer of an ONOstorage dielectric layer entails substantial oxidation of the nitridelayer, requiring temperatures in excess of 900° C. for times as long asfour hours, which represents a significant thermal load and process timeoverhead. In addition, during this long thermal process, oxygen speciescan diffuse to the tunnel oxide interface and cause defects. Conversely,the formation of the final upper oxide layer 266′ using the dilute steamambient oxidation 170 only requires the prior formation of an OHdiffusion path from the gate sidewalls to the center thereof. Hightemperatures need only be maintained long enough to generate an initialupper oxide layer 266 (FIG. 7B) with a minimal thickness of about onemonolayer (e.g., 6-10 Å). The remainder of the final upper oxide layer266′ (FIG. 10B) can then be added more quickly and without as high athermal load by dilute steam oxidation. Such a reduction of the thermalbudget equates to a substantial reduction in the number of charge trapsgenerated in the final tunnel dielectric layer 230′. Therefore, use ofthe dilute steam ambient oxidation 170 to fabricate the oxide layers ofthe gate electrode 285 yields devices with improved performanceproperties, as compared to devices fabricated using conventionaltechniques.

By providing additional oxide at existing oxide-silicon interfaces, thesteam ambient oxidation 170 contributes to the repair of defects atthese oxide-silicon interfaces, thereby improving the ultimate deviceperformance. For example, the elevated temperatures and long processtimes associated with the fabrication of the remaining components of thegate electrode 285 induce vacancies and dangling bonds in the initialtunnel dielectric layer 230. If left unrepaired, these defects can actas charge traps, degrading the ultimate performance of the flash device.However, by exposing the patterned gate electrode 285 to the dilutesteam ambient, OH radicals are able to diffuse to these defects to fillthe oxygen vacancies and tie up the dangling bonds in the region of thefinal tunnel dielectric layer 260′. Therefore, the defect density at thefinal tunnel dielectric layer 260′ is reduced, with a correspondingincrease in the performance of the flash memory device. In certainarrangements, the dilute steam ambient oxidation 170 is the last chanceto repair the defects at the oxide-silicon interfaces before the gateelectrode 285 is encapsulated in a liner layer, and a thick BPSGinsulating layer.

At the same time, where the gate electrodes comprise crystallinematerial like the preferred polysilicon, the dilute steam ambientoxidation 170 passivates the grain boundaries in the polysilicon layersof the gate electrode 285. FIG. 11 schematically illustrates these grainboundaries in a polysilicon layer at an oxide-polysilicon interface. Theerase speed in flash memory devices is observed to be a strong functionof the grain size (or equivalently, the grain number) of the floatinggate 250 in contact with the final tunnel dielectric layer 230′. Forsmaller grain sizes (or more numerous grains), the erase speed has beenobserved to be faster than for larger grain sizes (or less numerousgrains). This result indicates that the erasure charge transfer throughthe polysilicon occurs substantially along the grain boundaries,possibly via the numerous defects and interface states along these grainboundaries. Thus, processes which form patterned gate electrode 285 withwidely varying grain sizes may be subject to non-uniform erasure, andcorrespondingly varying flash memory device operation. The dilute steamambient oxidation 170 of the preferred embodiment of the presentinvention can passivate the grain boundaries of the polysilicon floatinggate 250 by forming oxide bonds along the grain boundaries, asschematically illustrated in FIG. 11, thereby eliminating grain size ornumber as an erase variable. Therefore, the dilute steam ambientoxidation 170 may be used in certain embodiments to achieve more uniformerasure characteristics among various flash memory devices.

In other embodiments, process parameters may be adjusted in order toinduce some amount of smile in the oxide layers formed during the dilutesteam ambient oxidation 170. Under some circumstances, a non-negligiblesmile of the final tunnel dielectric layer 230′ is advantageous to theultimate device performance. For example, such a smile structure of thefinal tunnel dielectric layer 230′ effectively rounds the edges andcorners of the polysilicon floating gate 250, thereby reducing theelectric field in these regions. As a result, better channel control andless hot electron degradation is achieved, and the threshold voltage ofthe flash memory device can be adjusted. For a dilute steam ambient with3.5% steam in 3 slm H₂, different temperatures and exposure timesproduce varying amount of smile in the final tunnel dielectric layer230′, as shown in Table 1:

Center Oxide Edge Oxide Time (Minutes) Temperature (° C.) Thickness (Å)Thickness (Å) 105 750 11 31 189 700 26 47 363 650 64 68

In the preferred embodiment of the present invention, the fabrication ofthe flash memory devices continues with subsequent processing steps, asillustrated in FIG. 12. Spacers 290 a and 290 b are formed along thesidewalls of the patterned gate electrode 285 after the dilute ambientoxidation. Conventional blanket deposition of an insulating materialfollowed by a directional spacer etch can be employed for spacerformation. Doping of the underlying silicon substrate 220, therebycreating the source and drain regions of the flash memory device, can beperformed after formation of the spacers 290 a and 290 b.

The gate electrode 285 and other surrounding areas are then covered by asubstantially conformal liner layer 292. The liner 292 comprises aninsulating material, preferably incorporating both silicon and nitrogen.Preferred liner materials include silicon oxide, silicon nitride,silicon oxynitride or a multiple layer laminate including one or both ofnitride and oxynitride. The liner 292 can be formed by any suitablemanner, but is preferably formed by chemical vapor deposition (CVD) toensure good step coverage over the topography of the patterned stackedgate structures 26 across the substrate 20.

Subsequent to forming the liner layer 292 in the preferred embodiment ofthe present invention, an interlevel insulating layer 294 is depositedover the structure. Typically composed of BPSG, the layer 294 serves toelectrically isolate underlying devices, such as the illustrated EEPROMtransistor. From overlying interconnects. Accordingly, the interlevelinsulating layer 294 is preferably between about 6,000 Å and 20,000 Å inthickness.

After depositing the interlayer insulating layer 294, the integratedcircuit is completed by additional fabrication steps. Typically, suchsteps include metallization processes, interconnecting various devicesof the integrated circuit. In order to make contact electrical contactbetween metal layers and the electronic devices, holes or vias areetched through the interlevel dielectric layers, such as the interlevelinsulating layer 294, and then filled with conductive material. Contactto the control gate 270 and active areas in the substrate 220, forexample, require contact through the interlevel insulating layer 294 andthe liner layer 292.

After metallization steps, the integrated circuit is then completed byformation of bond pads and final passivation, such as by deposition of afurther silicon oxynitride layer or other suitable passivation material.As will be appreciated by the skilled artisan, the passivation layerforms a seal against moisture or other corrosive agents.

Although described above in connection with particular embodiments ofthe present invention, it should be understood the descriptions of theembodiments are illustrative of the invention and are not intended to belimiting. Various modifications and applications may occur to thoseskilled in the art without departing from the true spirit and scope ofthe invention as defined in the appended claims.

We claim:
 1. A method of fabricating an integrated circuit, comprising:providing a silicon semiconductor substrate; forming a transistor gatestack comprising an oxide-silicon interface defined by an initial oxidelayer directly in contact with a silicon layer; patterning and etchingthe transistor gate stack to define a gate electrode that includes theoxide-silicon interface; and forming a final oxide layer with additionalsilicon oxide bonds in the region of the oxide-silicon interface byexposing the initial oxide layer and the gate electrode to elevatedtemperatures and a dilute steam ambient having a partial pressure ofsteam between about 1% and 50% of the dilute steam ambient.
 2. Themethod of claim 1, wherein the percentage of steam in the dilute steamambient is less than about 10%.
 3. The method of claim 1, wherein thepercentage of steam in the dilute steam ambient is less than about 5%.4. The method of claim 1, wherein the thickness of the initial oxidelayer is between approximately 5% and 95% of the thickness of the finaloxide layer.
 5. The method of claim 1, wherein the thickness of theinitial oxide layer is between approximately 40% and 60% of thethickness of the final oxide layer.
 6. The method of claim 1, whereinthe initial oxide layer is formed on a surface of the siliconsemiconductor substrate.
 7. The method of claim 6, wherein the finaloxide layer comprises a tunnel oxide in a flash memory device.
 8. Themethod of claim 7, wherein the thickness of the initial oxide layer isbetween approximately 6 Å and 94 Å.
 9. The method of claim 8, whereinthe thickness of the final oxide layer is between approximately 100 Åand 110 Å.
 10. The method of claim 7, wherein the thickness of theinitial oxide layer is between approximately 5% and 95% of the thicknessof the final oxide layer.
 11. The method of claim 10, wherein thethickness of the initial oxide layer is between approximately 40% and90% of the thickness of the final oxide layer.
 12. The method of claim11, wherein the thickness of the initial oxide layer is betweenapproximately 70% and 90% of the thickness of the final oxide layer. 13.The method of claim 1, further comprising forming a floating gate layerdirectly over the initial oxide layer, forming a storage dielectric overthe conductive layer, and forming a control gate layer over the storagedielectric prior to patterning and etching.
 14. The method of claim 13,wherein the floating gate layer comprises polysilicon.
 15. The method ofclaim 1, wherein the final oxide layer forms at least a part of astorage dielectric layer above a floating gate in a flash memory device.16. The method of claim 15, wherein the storage dielectric layercomprises an oxide-nitride-oxide construction.
 17. The method of claim16, wherein the thickness of the initial oxide layer is betweenapproximately 10 Å and 40 Å.
 18. The method of claim 17, wherein thethickness of the initial storage oxide layer is between approximately 15Å and 25 Å.
 19. The method of claim 18, wherein the thickness of thefinal oxide layer is approximately 40 Å.
 20. The method of claim 1,wherein the dilute steam ambient comprises a hydrogen-containingcompound.
 21. The method of claim 20, wherein the hydrogen-containingcompound comprises H₂.
 22. The method of claim 21, wherein thehydrogen-containing compound is selected from the group consistingessentially of NH₃, N₂H₄ and hydrazine.
 23. The method of claim 1,wherein the elevated temperatures are between approximately 500° C. and1,000° C.
 24. The method of claim 1, wherein the elevated temperaturesare between approximately 600° C. and 800° C.
 25. The method of claim 1,wherein the initial oxide layer is directly above a nitride layer. 26.The method of claim 1, wherein a difference between a maximum and aminimum thickness of the final oxide layer is between about 20 Å and 40Å.
 27. The method of claim 27, wherein a difference between a maximumand a minimum thickness of the final oxide layer is between about 4 Åand 8 Å.
 28. A method of fabricating an integrated circuit, comprising:providing a semiconductor substrate; forming a transistor gate stackcomprising an oxide-silicon interface defined by an initial oxide layerdirectly in contact with an overlying polycrystalline silicon layer;etching the transistor gate stack to define a gate electrode thatincludes the oxide-silicon interface; and passivating polycrystallinesilicon grain boundaries across the oxide-silicon interface by exposingthe grain boundaries to a dilute source of OH radicals, the dilutesource comprising less than about 10% steam.
 29. The method of claim 28,wherein passivating comprises maintaining a reaction temperature betweenabout 600° C. and 800° C.
 30. A method of fabricating an integratedcircuit, comprising: forming a plurality of layers over a semiconductorsubstrate, the plurality of layers including a buried oxide layer havingan interface with a silicon source layer; etching the plurality oflayers to expose a surface of the buried oxide layer; and diffusing OHspecies through the buried oxide across the interface to grow additionaloxide on the silicon source layers by exposing the plurality of layersto elevated temperatures and a dilute steam ambient having a partialpressure of steam between about 1% and 50% of the dilute steam ambient.31. The method of claim 30, wherein the buried oxide layer forms a partof an oxide-nitride-oxide structure, and forming the plurality of layerscomprises: growing an initial lower oxide; forming a silicon nitridelayer over the initial lower oxide; and oxidizing through a surface ofthe silicon nitride layer to form an initial upper oxide layer.
 32. Themethod of claim 31, wherein etching comprises defining a gate electrodefor an electrically erasable programmable read only memory device. 33.The method of claim 31, wherein the buried oxide layer comprises atunnel oxide in a flash memory device.
 34. The method of claim 31,wherein diffusing comprises exposing the etched plurality of layers tothe dilute steam ambient at between about 500° C. and 1,000° C.
 35. Amethod of fabricating an integrated circuit, comprising: providing asilicon semiconductor substrate; forming a gate stack comprising aburied polysilicon layer with grain boundaries at a polysiliconinterface; etching the gate stack to define a gate electrode thatincludes the buried polysilicon layer; and passivating the grainboundaries of the polysilicon layer by exposing the gate electrode toelevated temperatures and an ambient comprising a compound which yieldsOH radicals.